Method of treating oil wells



Patented Dec. 12, 1944 METHOD OF TREATING OIL WELLS Kurt H. Andresen, Bradford, Pa, assignor to Case, Pomeroy & Company, New York, N. Y., a

corporation of Delaware No Drawing. Application September 9, 1941, Serial No. 410,160

3 Claims. (Cl. 166-22) This invention relates to an improved method of selectively plugging permeable fluid-carrying strata exposed to an oil well, and more particularly the invention relates to an improved method of selectively plugging sand strata. exposed to a water or gas intake well which is part of a flooding or gas repressuring system, or to a producing well producing by a fluid drive or by a combination of fluid drive and pumping.

This application is a continuation-in-part of my co-pending application Serial No. 356,736. filed September 14, 1940.

In a water flooding or gas repressuring operation to produce oil from oil-bearing sands a fluid, such as water or a gas, is forced into an input well and through the oil-bearing strata to force 011 to flow through the strata to an output or producing well from which oil or a mixture of oil and the fluid is taken. In accordance with this process a series of input wells approximately equidistant from a producing well are drilled and exposed to the same oil-bearing strata. Fluid is forced down the input wells and out into the oilbearing strata under sufficient pressure to drive the oil through the strata into the output or producing wells.

In carrying out this method of oil recovery serious difliculties have been encountered when a stratum permits flow of driving fluid to the output well without forcing oil therefrom into the output well. This condition frequently arises because oil-bearing strata having different permeabilities and so difl'erent degrees of fluid conductivity may lie closely adjacent one another and be exposed to the same input well opening. Under these circumstances the driving fluid will drive the recoverable oil from the more permeable strata before the recoverable oil is driven from adjacent less permeable strata leaving substantial quantities of recoverable oil in the less permeable strata. As more permeable strata are emptied of recoverable oil they become more permeable to the less viscous fluid of the fluid drive and act to short-circuit the driving fluid operation. Thus. it becomes necessary to pump in more and more pressure fluid for each barrel of oil produced at the output well and a condition may be reached where the cost of pumping and handling the quantity of driving fluid necessary to produce a barrel of oil is greater than the value of the oil recovered.

Also, since the rate of oil production under fluid drive is. among other factors, a function of the conductivity of the sands and the pressure drop across them, it requires a higher pressure to maintain the desired rate of production from the tighter or less permeable oil-producing sands. But if, after the more permeable strata are depleted, the pressure of the driving fluid is increased to increase the production from the tighter sands, the flow of driving fluid through the loose sand may become so great as to render the production unprofitable.

Another problem that frequently arises in water-flooding or gas repressuring operations is caused by the presence of highly permeable strata having no producible oil but lying adjacent oil-producing strata. Such thief sands may take large amounts of the driving fluid which is entirely lost so far as oil production is concerned. oftentimes such loose streaks develop during the producing operations and it is difiicult to determine exactly where such streaks lie with reference to the oil-producing strata.

The process of the present invention provides in general that when the ratio of driving fluid to oil produced at the output well becomes unduly high due to depletion of more permeable strata adjacent less permeable recoverable oil-bearing strata, or due to the presence of "thief" sands, the depleted more permeable strata or thief sands are so selectively sealed, or their conductivity is so reduced. that when injection of driving fluid is resumed a greater proportion of the driving fluid will flow through the non-depleted oil-producing strata. Further, the seal pro- 7 vided is such that the pressure under which the pressure fluid is injected into the wells may be appreciably increased without increasing the driving fluid to oil ratio. Thus, the rate of oil production from remaining tighter (less permeable) oil-producing sands is increased.

The problems involved in thus selectively sealing sand strata located hundreds of feet below the surface of the ground are difficult. Methods of selective sealing utilizing the relative positions of the adjacent depleted and non-depleted strata and their respective depths below the ground surface are relatively unsatisfactory where the wells are deep and possibilities of error in the depth measurements are increased. Yet the selectivity of the sealing is necessary to prevent sealing of producing strata. Also, to oflset the necessity for successive treatments the dam formed by the sealing process must be permanent and able to withstand high pressures when the fluid drive is carried out at such high pressures.

Another characteristic of the well that makes se e tive sealing of the strata difflcult is that the walls of the well exposed to the producing strata are "shot" by setting oil a high explosive which ruptures, breaks, and produces ilssures in the sand extending undeterminable distances in irom the well hole proper and also increasing the effective diameter oi parts oi the well hole undeterminable amounts.

In the' resent embodiment of the invention these problems are solved by injecting into the input well a solution of a gelling material, which solution is caused to flow preferentially into the pores oi the more permeable strata. Then by a subsequent and controlled action the chemical and/or physical properties of the gelling material are so altered within the pores oi the depleted strata as to decrease the conductivity of the strata into which the treating solution is flowed. The gelation is accomplished by treating the solution of the gelling material with a chemically active material Just prior to injecting the solution into the pores of the strata to be sealed. The chemically active material and gelling material are so chosen that the eiiect oi the gelling action oi the selling material .is postponedadesiredlengthoitimeaiterthemixture is prepared.

More specifically, -a solution of sodium silicate and a precipitating agent such as sodium or ammonium bicarbonate is flowed into the input well in such manner that it selectively enters the pores of the oil-depleted more permeable strata. The bicarbonate solution serves to reduce the alkalinity of the sodium silicate solution in the desired manner and under this condition the sodium silicate solution proceeds to iorma hydrosol oi so-called silicic acid. This hydrosol, over a period oi time, changes into a hydrogel having a viscosity many times greater than that of the hydrosol. Th time required ior the formation of the hydrogel depends upon a number oi iactors, such as the pH oi the solution and the initial concentrations oi the reagents. For example, the formation oi the sell takes place more rapidly (for a given concentration of the sodium silicate) as the concentration of the sodium bicarbonate is increased. Also for a given concentration oi sodium bicarbonate, as the concentration oi the sodium silicate is decreased, the setting tim is increased. Thus, by making the concentration oi the sodium bicarbonate small enough, many hours may be required for the hydrogel to form. By suitably selecting the initial concentration of the reagents the interval required ior gelation under given conditions may be accurately predetermined. Thus, after the solution is selectively flowed into the pores of the strata to be sealed, a predetermined time interval elapses before any material gelatinising takes place. This delay allows suflicient time ior the injection of the treating solution into the well andinto the strata to be sealed.

In choosing the reagent to reduce the alkalinity oi the sodium silicate solution, as above described, care is taken to select one which does not react with the silicic acid to form a waterinsoluble silicate. One oi the advantages oi using sodium or ammonium bicarbonate as the reagent the bicarbonate may be varied considerably to adjust the gelling time over a relatively wide range and without risk oi causing such rapid gelling as to interfere with the introduction oi the treating solution into the pores of the strata to be sealed.

Thus, as the solution is selectively flowed into the pores of the strata to be sealed, no gelling takes place until some time after the injection is completed.

The fluid silicic acid gel that iorms in the pores of the rigid sand strata acts as a dam to the flow oi fluid therethrough since the flow oi the gel] itself through the pores is substantially zero even if large pressure drops across the gell iorm. That this fluid gel forms such a dam is shown by the equation oi Darcy's law which is as follows:

L=unit length.

This equation states that the flow through the sand structure varies directly with the pressure drop across it and inversely with the coeiiicient oi viscosity and the length oi the fluid flow. A

is that not only do these reagents not form an relatively dilute solution of the sodium silicate produces a gel whose viscosity in oentipoises may be in the order oi millions. Thus, by using only a dilute solution of sodium silicate (which has deflnite advantages) and forming the gell over a sumcient depth irom the well, the fluid gel forms a substantially immovable dam which is as eiiective as though it were rigid.

With regard to the question oi the concentration oi the treating solution that is introduced, the iollowing factors, among others, require consideration. The more concentrated the solution the more viscous the gel subsequently iormed irom it, and thereiore a concentration is picked suiiiciently high to obtain a sumciently viscous gel.

But since it is desirable to have the gel form only in the pores, and through a range extending to a sufllcient depth from the suriace oi the sand exposed to the well opening, the concentrations of the components of the solution is kept such that the solution freely flows into the more permeable pores and does not start gelling until suiilcient time is allowed ior it to iiow completely into the pores.

In the iollowingexample oi an application of the process oi the invention, reierence will be made to a water intake well oi a so-called wateriiooding oil-producing system. In such a system a number oi input wells are drilled around a producing or output well at points spaced approximately equidistant from the output well. Each of the input wells is provided with a pipe line extending irom the suriace of the ground through the usual packings to the oil-producing'strata exposed to the input well hole. Water is iorced down the pipe line and into the strata and causes oil therein to flow toward and into the output wells. The oil flowing into the output well is pumped out or showed to iiow out under its own head, and the same applies to water which may flow into the output well.

As this water-flooding proceeds, the oil is produced ilrst from the more permeable stratum exposed to the input and outputwells, that is, it is produced first from the stratum through which the oil and water flow with less resistance than they do through less permeable strata. When all the oil that can be economically produced by this method flows out of the more permeable stratum, the stratum offers less resistance to the flow of water and this "oil-depleted" stratum now acts to short-circuit the water flow so that if the same driving fluid pressure is maintained the water intake increases without increasing the oil production from the less permeable or tighterstrata. The relatively slow production from the tighter strata can be increased only by increasing the pressure of the water drive, which, however, automatically causes an increased rate of water ilow through the oil-depleted more permeable strata and so greatly increases the water intake without causing a corresponding increase in the oil-production.

' At this point the input well is treated in accordance with the present invention. As one speciilc embodiment, when the above condition arises, indicating that a more permeable stratum is depleted of its producible oil, the input water drive is shut oil and a mixture of sodium silicate and sodiumbicarbonate dissolved in water is flowed into the pores of the more permeable oildepleted strata. The manner in which this may be accomplished is as follows.

If the strata to be plugged is suillciently loose and has a low enough "rock pressure, the water in the well and the tubing may partly empty under hydrostatic pressure into the more permeable strata, But if such is not the case, the well is emptied of water by flowing compressed air into the top of the well to force water therein to ilow into the permeable strata. After this operation a mixture oi the sodium silicate and sodium bicarbonate in a water solution may be slowly flowed into the well under such regulated pressure that it selectively enters predominantly the pores of the oil-depleted more permeablestrata. The solution thus flowed into the well under the reduced controlled pressure has a natural tendency to enter the more permeable stratum because it oilfers appreciably less resistance to flow than does the less permeable oil-bearing strata. Also, since the viscosity of the treating solution is greater than that of water, the selectivity of the solution to enter only the oil-depleted more permeable strata is increased since, as we have seen, viscosity is one of the factors affecting the flow of fluids through permeable sand. In many cases controlled hydrostatic pressure of the solution may provide sufllcient head to force the solution into the pores of the oil-depleted stratum, but when such hydrostatic pressure is not sufllcient it may be supplemented by imposing a suitable gas or other pressure on the treating liquid to cause it to flow into the well at the desired selective slow rate.

The concentration of the sodium silicate used is sufficiently great so that the subsequent precipitating and gelling action will provide a sumciently viscous deposit in the pores entered by the treating solution to withstand the driving fluid pressures to which the dam" formed by the gel will be subjected.

The amount of the treating solution used is determined in accordance with the thickness" 0! the strata being selectively plugged or sealed and in accordance with the estimated extent to which the dam is to extend radially outward from the well. These latter factors are determinable -from the knowledge of the sand structure exposed to the well usually available from core analyses made during the drilling of the well.

As aspeciflc example 0! such a treatment of a loose sand stratum of a water intake well, in the Broadford Field of Pennsylvania the following procedure was found to produce satisfactory selective plugging of the loose non-oil-producing sand. In the particular well treated, after the water drive was shut oil, the water level in the well dropped under its own head so that no additional fluid drive was necessary to clear the well of the water in it. First a mixture consisting of 133 lbs. of a commercial solution of sodium silicate (26.7% SiOz, 8.9% NazO, 62.4% H2O by weight) dissolved in 870 lbs. of water and 4.7 lbs. of sodium bicarbonate dissolved in 62 lbs. of water was flowed into the well under its own gravity. This mixture, which has a concentrationIof 3.6% SK): and 0.45% sodium blcarbonate (by weight), will not gel in less than 'sothat the adjusted concentration of a subsequent treating mixture would not be changed appreciably by dilution with water. The well was cleared of this liquidby forcing air into the well to drive the first treating mixture into the sand.

Then a mixture consisting of a solution of 216 lbs. of sodium silicate (made up of the same proportions as before) dissolved in 1400 lbs. of water and a solution of 19 lbs. of sodium bicarbonate dissolved in 280 lbs. of water was flowed into the well under its own gravity, and air was again applied to the well to force treating liquid tending to remain in the well out into the pores. This solution, which has a concentration of 3.3% S102 and 1.0% NaHCOs (by weight), gels in 6-8 hours at the ground temperature which was about 58 F.

Then, because the permeable oil-depleted stratum was assumed to lie above the less permeable oil-bearing stratum, so that some of the treating liquid was left in the bottom of the well up to the level of the more permeable stratum, 1.0 lbs. of a highly concentrated solution of sodium hydroxide was poured into the well to render the mixture remaining in the well so alkaline as to be incapable of gelling. Since the well bore generally extends below the producing strata it is generally desirable to add the sodium hydroxide to increase the alkalinity to prevent gelling even though the stratum being plugged is known to lie at the bottom of the permeable strata.

The solution introduced into the pores was held in place by suitably controlling the static pressure in the well to prevent the solution from moving in the pores prior to or during the gelling action. Then. after sufficient time was allowed for the gelling action to take place, the water drive was resumed.

With the dilute concentration of the sodium silicate thus used the gel that forms has a suillciently high viscosity so that the dam formed is substantially permanent. The low concentration used has an advantage in that the time for gelling varies with variation in the pH so long as the pH is above or below a narrow range. Thus as the pH is reduced down to said narrow range the setting time is correspondingly reduced. In the narrow range change of pH amount or said bicarbonate salt to cause the solution to gel in 6-8 hours, and selectively flowing said second solution into the pores of the sand to be plugged, flowing gas into said well as before, holding said solutions in place in said pores until the gelling action takes place and forms a fluid gel extending a sufficient depth from the well and having a suflicient viscosity so as to form a substantially permanent dam to the flow of fluid therethrough.

3. The method of selectively plugging off the flow of fluid through non-productive more permeable porous rigid sand formations exposed to an oil well without plugging less permeable productive strata, comprising the steps of preparing a solution of sodiumsilicate of such concentration that variation of the pH of the solution through practically the entire range causes a variation in the setting time of the siiicic acid gel, incorporating in the solution an amount of a bicarbonate salt sufiicient to reduce the pH of the solution to the point where the solution will gel only after the lapse of a relatively long period of time, flowing said solution into the well under conditions of such slow controlled flow \of the sand to be plugged, holding said solutions in place in said pores until the gelling action takes place and forms a fluid gel extending a sufficient depth from the well and having a sumcient viscosity so as to form a substantially permanent dam to the flow of fluid therethrough. KURT H. ANDRESEN. 

